Malaria is the best known protozoal disease, caused by one of four species of the sporazoa type—Plasmodium falciparum, P. vivax, P. ovale, and P. malariae. It is one of the most common infectious diseases in at least 100 tropical and subtropical countries in Africa, Southeast Asia, and South America. According to WHO, one out of every seventeen people alive today will die from a disease transmitted by the bite of a mosquito. Worldwide, malaria infects 300-600 million people and kills about three million in a year. The increasing prevalence of multiple drug resistant strains of Plasmodium falciparum in most malaria endemic areas has significantly reduced the efficacy of current anti-malarial drugs for prophylaxis and treatment of this disease. Although drug resistance is a common problem in the treatment of most microbial infections, malaria and many neoplasms, the impact is more acute for malaria chemotherapy because of the limited number of clinically useful anti-malarial drugs.
Only six prescription drugs are available in the US for treating and/or preventing malaria: Atovaquone/proguanil, Doxycycline, Mefloquine, Primaquine, Chloroquine phosphate, and Hydroxychloroquine sulfate. All of them are discovered more than 40 years ago. Serious side effects are common. Primaquine is the only available causal prophylactics. It has a low therapeutic index. The use of chloroquine is limited because of the worldwide emergence of drug-resistant strains of P. falciparum and P. vivax. Proguanil has a short half-life and strains of P. falciparum resistant to proguanil are common. The newest antimalarial drug Mefloquine was developed in the late 1960s. It is initially reserved by WHO for use in regions where drug resistance to chloroquine is a serious problem. However, problems have arisen with mefloquine use. The cure rate for mefloquine-sulfadoxine-pyrimethamine treatment of P. falciparum in Southeast Asia fell from 96% in 1985 to as low as 50% in 1990. Mefloquine can also produce adverse neurological and psychiatric reactions. Artemisinin and its derivatives are currently under development. However, these compounds cannot be used during pregnancy since they have shown fetotoxicity in rodent models. The current generations of artemisinins possess poor efficacy of monotherapy. Artemisinins do not interfere with hepatic stages of parasite development and therefore have no causal prophylactic value.
Therefore, novel medicinal agents are urgently needed to overcome the emergence of resistance and to control an ever-increasing number of epidemics caused by the malaria parasite.
For decades, natural products have been a wellspring of drugs and drug leads. According to a recent survey, 61% of the 877 small-molecule new chemical entities introduced as drugs worldwide during 1981-2002 can be traced to or were inspired by natural products. These include natural products (6%), natural product derivatives (27%), synthetic compounds with natural-product-derived pharmacophores (5%), and synthetic compounds designed on the basis of knowledge gained from a natural product (that is, a natural product mimic; 23%). In certain therapeutic areas, the productivity is higher: 78% of antibacterials and 74% of anticancer compounds are natural products or have been derived from, or inspired by, a natural product.
Carica papaya L., a perennial tropical tree, belongs to the family of Caricaceae, and is commonly known as papaya, paw paw, kates, or papaw. It has an erect, branchless trunk with scars from old leaf stems. It is really only a very coarse and robust herb with leaves reaching sometimes nearly 3 feet across. Carica papaya L. is widely grown in tropical regions, including Central America, the Amazon region, and the Florida, Hawaii, Puerto Rico of the United States, for its edible fruit known as papaya. The fruit is high in vitamin A and is consumed fresh when ripe while the green fruits are grated in salads or boiled like squash. The green fruit of papaya is the source of the enzyme papain, which is used in meat tenderizers and many other biologically active phytochemicals. The leaves of papaya are divided into several lobes, which radiate like the fingers of the hand. In Suriname's traditional medicine, the boiled green leaves of papaya are used against malaria and as an anthelmintic, the seeds as a vermifuge and tea of the fallen leaves against hypertension. The decoction of the leaves has also been used as a treatment for intestinal parasites.
An ethanol extract of the dried leaves of Carica papaya L. exhibited considerable in vitro antimalarial activity to warrant fractionation. On the basis of the initial activity of crude extracts, attention was focused on the bioactivity-guided fractionation of the EtOH extract of the dried leaves, which resulted in the isolation of two new antimalarial natural products, Formula A and Formula B. Their chemical structures are shown in FIG. 1. Formula A, bearing two secondary amine moieties, was converted into the corresponding di-acetamide compound Formula I. Formula B, also bearing two secondary amine moieties, was converted into the corresponding di-acetamide compound Formula II. The chemical structures of the natural product derivatives, Formula I and Formula II, are shown in FIG. 2. The synthesis route is shown in FIG. 3.
Two Plasmodium falciparum malaria parasite clones, W2 (chloroquine resistant) and D6 (chloroquine sensitive), were utilized for in vitro efficacy testing. All new compounds were also tested for toxicity against human adult liver epithelial cells (THLE-3). Natural products Formula A and Formula B showed modest inhibitory activity against both chloroquine sensitive malaria strain (D-6) and chloroquine resistant malaria strain (W-2). The two derivative compounds Formula I and Formula II showed equally potent inhibitory activity against both chloroquine sensitive malaria strain (D-6) and chloroquine resistant malaria strain (W-2). The IC50's (concentration of compound that affords 50% of inhibition) were superior to the positive controls, chloroquine and mefloquine. Noticeably, these compounds also showed much reduced toxicity.
In mice models, both Formula I and Formula II have shown excellent blood schizonticidal activity and oral prophylactic activity.
The present invention relates to new, more active and less toxic natural product derivatives for the treatment of malaria.